Generally, when processed soybean products such as tofu, soymilk, and soybean casein (yuba) are produced, an operation to process raw soybeans and change them into a liquid form is performed.
This operation comprises: an immersion step in which raw soybeans that have been thoroughly washed are immersed in water for approximately one night until they swell to approximately twice their size; a grinding step in which a slurry is obtained by rubbing and crushing the swollen raw soybeans while adding an appropriate amount of water; and a heating step in which the soybean slurry undergoes thermal denaturation by being heated to a predetermined temperature. If required, a separation step in which solid pulp (okara) is separated may also be performed either before or after the heating step. In addition, it is also possible to omit the immersion step in cases such as when soymilk is being produced for human consumption.
By further processing the soybean slurry obtained through each of the above processes, processed soybean food products are produced. For example, tofu can be obtained by adding a coagulant to the soybean slurry so as to coagulate it.
In this type of processed soybean food products production method, the heating step renders harmless the harmful substances contained in the soybeans by the heating thereof and also simplifies the soybean protein and changes it into a state in which it can be easily digested. The heating step is thus particularly important (see page 121 of “Soymilk”, by Kanji Tsuchiya, Food Research Institute, 1980).
The heating step can be separated into a temperature raising step in which the soybean slurry is heated so as to raise the temperature thereof and a thermal denaturation step in which the soybean protein is thermally denatured by maintaining the heated soybean slurry at a raised temperature for a predetermined time. However, normally, because of the type of heating apparatus used or because of the operating conditions, the soybean slurry is thermally denatured while the temperature thereof is being raised, and there is usually no distinction between the temperature raising step and the thermal denaturation step.
However, the nature of soybean protein is such that it is extremely susceptible to being permeated with air. Therefore, if the soybean slurry becomes permeated with a large amount of air in the grinding step and a large number of air bubbles become mixed in the soybean slurry, then this has a deleterious effect on the final product.
For example, if tofu is produced using soybean slurry in which air bubbles have become mixed, the finished tofu has insufficient hardness and the constituent elements of the soybeans are oxidized by the oxygen in the air bubbles resulting in the problem arising that an unpleasant raw odor is given off.
Therefore, technology for removing air bubbles from the soybean slurry using a deaerating apparatus is known as a method for solving the above types of problem. Examples of these are the technology disclosed in Japanese Unexamined Patent Application, First Publication (Kokai), No. Sho 52-54069 in which air bubbles are removed from the soybean slurry by deaeration using a deaeration apparatus before the heating step is performed, and the technology disclosed in Japanese Unexamined Patent Application, First Publication (Kokai), No. Sho 61-195660 in which a deaeration process is performed after the heating step.
However, because the soybean slurry is in a liquid-slurry form, the removal of air bubbles is extremely difficult, particularly when the temperature thereof is low which causes the viscosity to increase. Accordingly, as is disclosed in Japanese Unexamined Patent Application, First Publication (Kokai), No. Sho 52-54069, when the deaeration process is to be performed on the slurry that is at low temperature prior to the heating step being performed, the removal of the air bubbles is exceedingly difficult and it is necessary to perform the deaeration using strong suction provided by an apparatus with a complicated structure. Furthermore, because the deaeration is difficult, it is not possible to remove minute air bubbles, which has resulted in the problem arising that an unpleasant raw odor caused by the air bubbles remains in the finished product.
Moreover, after the heating step is completed, the soybean slurry absorbs the odor already generated in the soybean protein in which the thermal denaturation is well advanced and this odor becomes fixed in the soybean slurry. Therefore, as is disclosed in Japanese Unexamined Patent Application, First Publication (Kokai), No. Sho 61-195660, the problem arises that the odor has already been generated in the soybean slurry even if the air bubbles are removed from the soybean slurry that has undergone the heating step. Namely, once the odor has been generated, there has not been much point to then performing the deaeration and a satisfactory effect as regards odor removal has not been obtained.
In this way, in either of the conventional technologies in which the deaeration process is performed on the soybean slurry before or after the heating step, it has not been possible to obtain a satisfactory deodorization effect and it has thus not been possible to obtain high quality soybean food products.
Furthermore, there has not hitherto existed a thermal deaeration apparatus that combines both the function of heating the soybean slurry and the function of deaerating the soybean slurry, and the development of an apparatus that efficiently provides high quality soybean food products has long been desired.